1. Technical Field
The present invention relates to an electrically switchable optical device operating in transmission for manipulating an incident light wave or light waves passing through the device. The invention also relates to a method for forming electrically switchable optical devices for forming such an optical switching device.
2. Discussion of Related Art
Optical signals in different forms are today increasingly utilized in many different types of devices and applications. In order to take full advantage of systems including optical signals or beams, it must be possible to direct the optical signal or beam coming in on a guided optical conduit, or on some other type of optical system in a desired electrically controlled manner to another optical conduit or to another optical system. The aforementioned optical conduit can be, for example, an optical fiber or other type of optical waveguide. There exists a wide variety of optical systems, which work under fast changing operational conditions, and thus require capability to perform optical functions in an efficient and electrically controlled manner.
Especially the recent rapid development of optical telecommunication and optical data processing systems creates increasing needs for versatile electrically switchable optical devices.
In addition to the act of simply switching the optical signal/beam on or off, the term “optical switching” above and hereinbelow also refers to more complex optical functions, i.e. transformations of the optical signal/beam and/or its path. These include, for example, dividing, redirecting or modulating the amplitude or phase of the optical signal/beam in a desired manner.
In the following, some prior art solutions for electrically controlled optical switching are shortly discussed. However, methods which are based on first converting optical signals into electrical signals for switching and then reconverting said electrical signals back into optical signals for outputting are not included in the following discussion as they are not relevant to the present invention.
A conventional method for electrically controlled optical switching is to mechanically move the optical components, for example mirrors, beamsplitters or beam attenuators in order to affect the propagation of the optical signal/beam. Said mechanical movements can be realized using various kinds of electrical actuators. However, such optical components together with the required electrical actuators cannot be easily made very compact in size and they are also rather difficult and expensive to manufacture, especially as mass-produced articles.
Silicon-surfacemicromachining is a recent technology for fabricating miniature or microscopic devices. This technology has also been used for manufacturing optical microelectromechanical systems (optical MEMS).
U.S. Pat. No. 5,867,297 discloses an oscillatory optical MEMS device including a micromirror for deflecting light in a predetermined manner. Small physical sizes and masses of these micromachined silicon “machine parts” make them more robust and capable of faster operation than conventional macroscopic mechanical devices.
Grating Light Valve™ devices by Silicon Light Machines, USA represent another type of optical MEMS devices. U.S. Pat. No. 5,311,360 discloses a light modulator structure, which consists of parallel rows of reflective ribbons. Alternative rows of ribbons can be pulled down by electrostatic attraction forces a distance corresponding to approximately one-quarter wavelength to create an electrically controlled grating like structure, which can be used to diffractively modulate the incident light wave. The electrical switching of the ribbons can be realized by integrating bottom electrodes below the ribbons, and by applying different voltages to the ribbons and said bottom electrodes to create the required electrostatic forces. U.S. Pat. No. 6,130,770 discloses another type of solution, where instead of using physical electrical connections to charge the predetermined ribbons of the light modulator structure, selected ribbons are electrically charged with an electron gun.
In principle, silicon optical MEMS technology uses processing steps derived from the integrated circuit (IC) fabrication techniques of photolithography, material deposition and chemical etching to produce the movable mechanical structures on a silicon chip. The aforementioned manufacturing process is, however, fairly difficult and thus expensive. Further, the optical MEMS devices operate mainly only in reflection and thus the capabilities of such devices of more complex transformations of the optical signal/beam and/or its path are limited. Material fatigue may also become significant in certain applications.
Birefringence, also known as double refraction, is a property which can be found in some transparent materials, for example in crystals. Such optical materials have two different indices of refraction in different directions. This can be used to create Pockels effect, an electro-optical effect in which the application of an electric field produces a birefringence which is proportional to the electric field applied to the material. The Pockels effect is well known in the art and it is commonly used to create, for example, fast optical shutters. However, because the use of birefringence requires use of polarized light, this severely limits its use as a general method in realizing optical switching devices.
U.S. Pat. No. 5,937,115 describes switchable optical component structures based on a holographic polymer dispersed liquid crystal. These are electronically controlled Bragg grating structures which allow to electronically switch on and off the diffractive effect of the transparent grating structures, which have been optically recorded or otherwise generated in the material. These electronically switchable Bragg grating (ESBG) devices can be used for various filtering or lensing applications. The major drawback of the ESBG technology is the complex manufacturing process required. Environmental concerns and hazards generally related to liquid crystal materials apply also to the ESBG devices.
U.S. Pat. No. 4,626,920 discloses a semiconductor device, which has an array of spaced charge storage electrodes on semiconductor material (Si) and an elastomer layer disposed on said electrodes. At least one conductive and light reflective layer is disposed over the elastomer layer. When voltages are applied between the charge storage electrodes and the conductive layer, this causes the deformation of the conductive/reflective layer and the elastomer layer from a flat surface to a form having a sinusoidally cyclically varying cross-section. Thus, the reflective front surface of the conductive layer can be utilized as an electrically switchable reflective grating.
GB patent 2,237,443 describes another light modulating device, where a reflective elastomer or viscoelastic layer is utilized for light modulation. In this arrangement an electron gun (cathode ray tube) is used instead of direct electrical connections/electrodes (cf. U.S. Pat. No. 4,626,920) to generate the electrical pattern needed to deform the elastomer layer.
An important aspect in the above described type of systems (U.S. Pat. No. 4,626,920 and GB 2,237,443) is the operation of the conductive/reflective layer or layers which is/are mounted on the deformable elastomer layer. Said conductive/reflective layer or layers must reliably and repeatably provide precise patterns of deformations which correspond to the charge pattern modifying the elastomer layer. This, together with the fact that said devices operate only in reflection, limits the use of such devices due to the limited selection of suitable conductive and reflective materials as well as due to the overall response characteristics (sensitivity to the applied voltages/charges, temporal response characteristics) of the device.
Yury P. Guscho “Physics of Reliofography” (Nauka, 1992, 520 p. in Russian) describes in chapter 7 a number of light modulator structures, in which a transparent viscoelastic layer is electrically deformed to manipulate the light passing through said viscoelastic layer. These devices can be taken to present the closest prior art with respect to the current invention, and they are therefore shortly described below with reference to the appended FIGS. 1a and 1b. 
FIGS. 1a and 1b correspond to FIG. 7.1 in chapter 7 of “Physics of Reliofography” and show the two basic schemes of the light modulator structures.
In the first scheme in FIG. 1a, the driving signal (U) for deforming the viscoelastic layer G is applied from the free side of the viscoelastic layer G using driving electrodes ES1, which electrodes ES1 are formed on the lower surface of a top glass substrate SM1. A gap is left between the free surface of the viscoelastic layer G and the lower surface of the top glass substrate SM1, allowing the viscoelastic layer G to deform without contacting the opposite structure. The aforementioned gap can be for example air, gas or vacuum. The electric field deforming the viscoelastic layer G is generated between the driving electrodes ES1 and the conductive substrate electrode ES2.
In the second scheme in FIG. 1b, the viscoelastic layer G is disposed on the driving electrode structure ES1, which in turn is formed on a glass substrate SM1. The electric field deforming the viscoelastic layer G is generated by applying alternating voltages to the neighbouring electrode zones in the driving electrode structure ES1.
In both of the aforementioned schemes, the free surface of the viscoelastic layer G can be coated with a conductive reflecting layer (sputtered metal film).
According to our best understanding, all the light modulator structures presented in the chapter 7 of “Physics of Reliofography” and discussed shortly above are based on the basic idea of deforming the viscoelastic layer into a surface structure having a substantially sinusoidally varying cross-section. This allows to use the viscoelastic layer as an electrically controlled sinusoidal grating in order to modulate the incident light wave.